Free electrons in metal nanostructures can be driven by light to oscillate collectively. This occurs at optical frequencies and results in so-called plasmon resonances. Plasmon resonances in these nanostructures are useful in applications where light needs to be coupled into or out of nanoscale materials. For example, plasmon resonances are used in sensing trace amounts of chemicals, heat assisted magnetic recording and solar cells. In these applications, the strength of the plasmon resonances is important. In other words, longer oscillation of electrons, i.e. a higher Q factor, is preferred. Accordingly, reduced damping in the electron oscillations is preferred to obtain such improved performances.
Plasmon resonances are strongly dependent on the size and shape of the nanostructures. Accordingly, lithographic methods that provide excellent size and shape control are preferred. However, these methods, when combined with metal deposition techniques such as electron-beam and thermal evaporation, result in polycrystalline nanostructures. The presence of grain boundaries in these polycrystalline nanostructures results in increased damping of the electron oscillations when compared with monocrystalline nanostructures and, therefore, leads to weaker resonances, i.e. a lower Q factor.
Smooth nanostructured surfaces are also desired to reduce further damping effects by surface scattering of electrons. Accordingly, deposition techniques that result in small grain size, e.g., deposition at low temperatures and/or high rate, are preferred. However, the smaller grain size in turn causes increased damping due to grain boundary scattering.
Annealing by thermal treatment is commonly used to improve crystallinity, such as promote crystal growth and minimize grain boundary defects in materials, and, thereby, increase Q factor. However, when applied to nanostructures on substrate, annealing also induces an undesired reshaping of the geometry, such as via dewetting of metals to form clumps. This reshaping is especially problematic in linear high-aspect structures such as the commonly-used nanorod antennas, as structural Raleigh instability results in segmentation of the nanostructure into randomly-sized particles.
Thus, what is needed is a process which preserves the original shape of the nanostructures while allowing grain boundary migration within the nanostructures. Furthermore, other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background of the disclosure.